70 THE FETAL MEMBRANES AND EARLY HUMAN EMBRYOS 



In the same way nutritive substances and oxygen must pass from the maternal 

 blood through these layers to enter the allantoic vessels. This exchange does take 

 place, however, and thus in Ungulates the allantois has become important not 

 only as an organ of respiration and excretion but as an organ of nutrition. Through 

 its vessels it has taken on a function belonging to the yolk sac in birds, and we 

 now see why the* yolk sac becomes a rudimentary structure in the higher mam- 

 mals. Excreta from the embryonic kidneys are passed into the cavity of the 

 allantois which is relatively large. The name is derived from a Greek word mean- 

 ing sausage-like, from its form in some animals. The chorion is important only 

 as it brings the allantois into close relation to the uterine wall, but in man we shall 

 see that it plays a more important role. 



UMBILICAL CORD 



Pig Embryos. — In their early development the relation of the amnion, allan- 

 tois, and yolk sac to each other and to the embryo is much the same as in the 

 chick of five days (Fig. 71). With the increase in size of the embryo, however, 

 the somatopleure in the region of the attachment of the amnion grows ventrad 

 (Fig. 70 D). As a result, it is carried downward about the yolk sac and allantois, 

 forming the umbiHcal cord (cf. Fig. 241). Thus in a pig embryo 10 to 12 mm. 

 long the amnion is attached at a circular Kne about these structures some distance 

 from the body of the embryo (cf. Fig. 119). The coelom at first extends ventrad 

 into the cord, but later the mesodermal layers of amnion, yolk stalk, and allantois 

 fuse and form a sohd cord of tissue. This is the umbilical cord of fetal life and its 

 point of attachment to the body is the umbilicus or navel. The cord is covered by 

 a layer of ectoderm continuous with that of the amnion and of the embryo and 

 contains, embedded in a mesenchymal (mucous) tissue, (1) the yolk stalk and (in 

 early stages) its vitelhne vessels; (2) the allantoic stalk; (3) the allantoic vessels. 

 These latter, two arteries and a single large vein, are termed, from their position, 

 the umbilical vessels. At certain stages (Figs. 122 and 123) the gut normally 

 extends into the ccelom of the cord, fonning an umbihcal hernia. Later, it re- 

 turns to the ccelom of the embryo and the cavity of the cord disappears. The 

 umbilical cord of the pig is very short. 



Human Umbilical Cord. — This develops like that of the pig and may attain a 

 length of more than 50 cm. It becomes spirally twisted, just how is not known. 

 In embryos from 10 to 40 mm. long the gut extends into the ccelom of the 

 cord (Figs. 179 and 180). At the 42 mm. stage, according to Lewis and Mall, 

 the gut returns to the coelom of the body. The mucous tissue peculiar to tlie cord 



